1. Field of the Invention
This invention relates to a method for stimulating the formation of cleats in subterranean coal formations and thereby enhancing the relative permeability of existing cleats to both water and gas via chemical treatments. The formation of conductive and contiguous cleat systems within a coal seam is enhanced when the coal matrix is contacted with an oxidative gas. The formation of highly conductive cleat systems allows effective and timely de-watering of the coal formation and increased methane permeability and production and increased ultimate methane recovery. The invention is particularly applicable in coal gas producing areas where production is limited by poor cleat development. The formation of the cleat system also facilitates the completion of wells by cavitation.
2. Brief Description of the Prior Art
Substantial quantities of methane gas are found in subterranean coal formations.
A variety of processes have been used in attempts to recover the methane from the coal formations more efficiently.
The simplest process is the pressure reduction process wherein a borehole is drilled into a coal formation from the surface and methane is withdrawn from the borehole by reducing the pressure to cause methane to be desorbed from and flow from the coal formation into the borehole and to the surface. This method is not efficient because coal formations are generally not extremely porous and the methane is generally not found in the pores of the coal formation but is absorbed onto the coal. While methane can be produced from coal formations by this process, the production of methane is relatively slow.
In some coal formations, the natural permeability is sufficient to allow the removal of in situ water to permit the enhanced recovery of methane. In such formations, cleat systems developed during the coal bed diagenesis provide channel ways through which water and methane migrate to the production wells for removal. This removal of water or "de-watering" of the coal formations removes water from the channel ways and permits the flow of methane through the channel ways and to a production well at a greater rate.
Many coal formations do not have extensively developed cleat systems or have cleat systems which are not fully developed. These coal formations have very low permeability to water and gas and do not yield water or gas at significant rates. As a result, the water fills the channels, and the recovery of methane from such coal formations at significant rates is difficult or impossible. Such low permeability water-containing coal formations may be either water saturated or less than fully water saturated. It appears that coal formations with better developed cleat systems may have been exposed to a diffusive oxidizing fluid of some type during the geologic past whereas coal formations with less developed cleat systems do not show evidence of exposure to an oxidizing fluid in the geologic past.
Accordingly, continuing efforts have been directed to the development of methods for replicating the effects of the conditions in the better developed cleat system coal formations during the geologic past.